The overall goal of this research program is to elucidate, at a biochemical level, the mechanisms involved in the control of pituitary hormone synthesis and release. Particular emphasis is given to the function of hypothalamic releasing factors in the regulatory process, and to the significance of feedback inhibition by end product hormones. These investigations utilize both primary cultures of normal pituitary cells and established clonal lines of pituitary tumor cells. In order to study the localization of receptors for thyrotropin-releasing hormone (TRH) on viable cells, a fluorescently labeled biologically active TRH derivative is employed. Photoaffinity labels for TRH receptors are being developed as an aid to the eventual purification and characterization of the TRH receptor. The mechanism of thyroid hormone action in pituitary cells is being studied; in GH3 tumor cells, L-triiodothyronine stimulates growth hormone, inhibits prolactin and regulates TRH receptors. The role of nuclear thyroid hormone receptors in these actions is being studied with emphasis on the way in which thyroid hormone reaches the nucleus.